A PBX connects a number of private extensions through a trunk line or lines to the public phone system, routing outbound and incoming calls and providing features such as voicemail and teleconferencing.
IP-PBXs work with VoIP telephones and trunks to provide these voice services on an IP-based data network. IP-PBXs can also mix IP and traditional telephony--for example, routing an incoming analog call from the PSTN to a VoIP telephone, or trunking a call /P>from an analog phone over the Internet.
By combining the Analog Devices Blackfin, open hardware, Asterisk, Oslec and uClinux, an extremely low-cost yet powerful IP-PBX reference design has been developed and put into production. The IP04 is being produced commercially and is also being adopted by a number of equipment suppliers as the engine for their IP-PBX products.
http://www.eetimes.com/news/design/showArticle.jhtml?articleID=202800249